Displacement conversion mechanism and actuator

ABSTRACT

A mechanism converts rotational displacement to linear displacement with ramped driver discoidal elements and ramped driven discoidal elements on a common central axis. The ramped surfaces of the two elements are complementarily shaped and opposed so that, when in contact and completely interengaged, they form an assembly of minimum length. The driver elements are rotated by an external force. The driven elements are allowed to translate along the common axis while being prevented from rotating about the common axis, whereby a rotational displacement of the driver elements by an externally applied force causes the elements to separate by camming action of the interengaged ramp surfaces to produce a linear displacement of the driven elements. A spring is coupled to the driven elements so as to restore the assembly to its minimum length in the absence of the externally applied force.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to displacement conversion mechanisms and,in particular, to the conversion of rotary to translational displacementand to actuators employing such mechanisms.

BACKGROUND OF THE INVENTION

Actuators for producing a mechanical displacement of a member to bedriven are employed throughout industry in a wide variety ofapplications. These include machinery control mechanisms, includingvalves and linkages, robotics, prosthetics, camera optics, pumps, brakesand power tools to name but a few. The displacement required may berotary, linear or other translational and of short or long stroke. Itmay be unidirectional, with a separate return mechanism such as a springor bidirectional, including reciprocation. The choice of actuator for aparticular application often depends on the environment in which it isto be used.

Many forms of actuator for producing linear or other translationaldisplacement of a driven object are known in the prior art. Theseinclude straightforward pneumatic and hydraulic piston arrangements andmore recently developed devices known as “air muscles” in whichinflation of a bladder causes contraction of an outer metal sheath in amanner similar to living muscle contraction. Other forms of linearactuator are electromagnetic, such as the solenoid and the voice coilmotor. Such devices have limited extension capabilities.

Electric motors, such as stepper or servo motors, are also convenientdrivers for actuator devices but to produce linear displacements theirrotary output must be transformed into a linear motion by a suitableconversion mechanism. Many such mechanisms have been employed for thispurpose such as the rack and pinion mechanism and the lead screw. In thelatter case, a short threaded nut is translated along a long threadedshaft rotated by the motor and is coupled to a member to be driven, suchas a print carriage. By appropriate choice of thread pitch or use ofadditional gearing, the mechanical advantage of this type of mechanismcan be increased to produce relatively large extensions for smallrotations.

Cam shafts and followers, biased by a return spring, are also widelyused, especially in conventional engines, for producing reciprocatinglinear motion and similar cam follower and spring arrangements are alsoused in power tools to produce a reciprocating action from aconventional electric motor drive shaft.

There is still scope however for a simple rotary to translational motionconversion mechanism, capable of producing large extensions for alimited angle of rotation and robust enough to be tolerant of hostileenvironments. The present invention offers such a mechanism.

Also known in the prior art are adjustable shims or spacer arrangementsfor producing a desired static linear displacement by relative rotationof complementarily shaped discoidal wedges or cams. Such arrangementsare described in U.S. Pat. No. 4,433,879 (J. C. Morris) for an“Adjustable Extension-Cam Shim” and in GB published patent application2331568 (A. Szmidla) for “Wedges and arrangements thereof”.

DISCLOSURE OF THE INVENTION

Accordingly, the invention provides a rotational to linear displacementconversion mechanism comprising: an assembly including a plurality ofdriver discoidal elements and a plurality of driven discoidal elementsmounted alternately on a common central axis to form an interleavedstack, each discoidal element having a ramped surface, the rampedsurfaces of adjacent elements being complementarily shaped and opposedso that, when in contact and completely interengaged, they form a stackof minimum length; coupling means for coupling the driver discoidalelements for rotation together about the axis by an externally appliedforce while permitting them to translate along the axis; said drivenelements being mounted in such a way as to permit translation along thecommon axis while preventing rotation of the driven elements about thecommon axis, whereby a rotational displacement of the driver elements bysuch an externally applied force causes the elements to separate bycamming action of their interengaged ramp surfaces so as to produce anextension of the stack corresponding to the cumulative separations ofthe driver and driven elements; and resilient bias means for restoringthe assembly to its minimum length in the absence of the externallyapplied force.

Such devices are very compact and rugged and, in contrast to the priorart devices of U.S. Pat. No. 4,433,879 and GB 2331568 which areessentially static and have no guide system or return mechanism, aresuitable for many dynamic precision applications such as positioningactuators or measured stroke fluid delivery devices, such as syringesfor medication or for fuel dispensers. Reciprocation may also beproduced by continuous rotation and used in pump applications.

Using a stack of elements allows for a much greater, cumulativeextension for a given rotation and is made possible by the coupling ofthe driver elements for rotation while allowing their linear separation.

This is preferably implemented by providing at least the drivendiscoidal elements intermediate the ends of the stack with axiallyaligned bores, each driver element having a projection extending axiallyfrom one face which extends through the bore of its adjacent driven discand locates in a recess in a proximate driver element in keyed,slideable engagement therewith so that rotational drive force can betransmitted between driver elements while allowing relative slidingmotion in an axial direction.

Preferably each said driver element recess is part of a bore through thedriver element and said projection is preferably part of at least onerib formed on the inner surface of the bore of its corresponding driverelement, which rib projects outwardly from its driver element discoidalportion and engages at least one complementarily oriented rib portion inthe bore of the proximate driver element to provide said keyed slideableengagement.

Although other arrangements would be possible, one preferred arrangementis for the bore in each intermediate driver element to be provided withtwo diametrically opposite ribs each extending over a 90 degree arc ofthe bore, said ribs being keyed into engagement with a similar pair ofribs in a proximate driver element oriented at 90 degrees to the firstmentioned pair of ribs.

The preferred way of preventing rotation of the driven elements is toprovide them each with a plurality of peripheral lugs, the mechanismfurther including grooved guide means surrounding the stack in which thelugs locate to prevent rotation.

Preferably, a driver element is located at one end of the stack and hasan outer surface adapted to be coupled to an external drive and an innerramped surface and a driven element is located at the opposite end ofthe stack and has an outer surface adapted to deliver a translationalload force and an inner ramped surface, intermediate driver and drivenelements having ramped surfaces on both sides. Preferably the end driverelement is fixedly mounted on an outwardly extending axial shaft,threaded externally for coupling to the external drive.

In such an arrangement, it is preferred that the mechanism includes ahousing assembly for the stack, comprising a cylindrical cover to oneend of which the terminal driven element is fixed, the other end of thecover terminating in a slotted flange. The housing assembly furthercomprises a fixed cage structure surrounding the cylindrical cover andbeing formed with a plurality of guide legs extending in the axialdirection and passing through the slots in the flange of the cylindricalcover to constrain it to linear movement. Additionally, the cylindricalcover is provided with external grooves and the guide legs are providedwith internal grooves in both of which said peripheral lugs locate, inoperation, to restrain the driven discs against rotation whilepermitting translation.

Another preferred feature is that the resilient bias means is a coilspring trapped between the flange of the cylindrical cover and an end ofthe cage.

Another preferred feature is that the driver and driven elements eachhave a plurality of ramps per ramped surface, distributedcircumferentially at evenly spaced positions. This enables an evengreater ratio of displacement to angle of rotation than would be thecase with a single 360° ramp.

For a single stroke application, it is preferred that the camming rampsurfaces are planar, rising at a relatively shallow angle to the planeof the discoidal elements and alternating with relatively steep returnsurfaces.

For a continuously rotated application, both the rising and fallingsurfaces of the ramps could be at the same angle or the ramped surfacesare smoothly undulating in form without discontinuity at the peaks. Thelatter arrangement is the more compact, in its unextended state.

For single stroke applications, the mechanism may include a stop forpreventing rotation of the driver element beyond the arc defined by theramp surface.

When provided with a drive mechanism for rotatably driving such driverelements, the displacement conversion mechanism becomes an actuator. Thedrive mechanism may be a motor or a manually operated crank. Acontinuously rotated driver element will produce a reciprocating linearoutput.

Such an output from a displacement mechanism including a rotatable crankfor rotatably driving the driver elements is eminently suitable for ahand pump application which would require a sealed casing for enclosingthe mechanism and forming a pump chamber containing a one way inletmeans for permitting fluid to be drawn into the pump chamber as themechanism contracts and an outlet for enabling fluid to be expelled fromthe pump chamber as the mechanism extends one way, as the crank isrotated continuously.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only,with reference to preferred embodiments thereof as illustrated in theaccompanying drawings, in which:

FIG. 1 is a side elevation of an unextended stepped disc stackillustrating the principles underlying a displacement mechanismaccording to the invention;

FIG. 2 is an end elevation of one of the discs making up the stack ofFIG. 1;

FIG. 3 is a side elevation of the disc stack of FIG. 1 in a partiallyextended state;

FIG. 4 is a side elevation of the disc stack of FIG. 1 in a fullyextended state;

FIG. 5 is an exploded view of an assembly of driver and driven steppeddiscs forming part of a displacement mechanism according to theinvention;

FIG. 6 is an enlarged exploded view of two driver and one driven disc atone end of the assembly of FIG. 5;

FIG. 7 is an isometric perspective view of the assembly of FIG. 5 in itsunextended state;

FIG. 8 is an isometric perspective view of the assembly of FIG. 5 in apartially extended state;

FIG. 9 is an isometric perspective view of the assembly of FIG. 5 in itsfully extended state;

FIG. 10 is a side elevation of a portion of an unextended undulatingdisc stack illustrating the principles of an alternative displacementmechanism according to the invention;

FIG. 11 is an isometric perspective view of one of the undulating discsmaking up the stack of FIG. 10;

FIG. 12 is a side elevation of the stack of FIG. 10 in a partiallyextended state;

FIG. 13 is a side elevation of the stack of FIG. 10 in a fully extendedstate;

FIG. 14 is an exploded view of an assembly of driver and drivenundulating discs forming part of a displacement mechanism according tothe invention;

FIG. 15 is an enlarged exploded view of driver and driven discs at oneend of the assembly of FIG. 14;

FIG. 16 is an isometric perspective view of the assembly of FIG. 14 inits unextended state;

FIG. 17 is an isometric perspective view of the assembly of FIG. 14 in apartially extended state;

FIG. 18 is an isometric perspective view of the assembly of FIG. 14 inits fully extended state;

FIG. 19 is an exploded perspective view of an actuator and displacementmechanism according to the invention including either the stepped discassembly of FIGS. 5 to 9 or the undulating disc assembly of FIGS. 14 to18;

FIG. 20 shows the actuator and displacement mechanism of FIG. 19 in itsunextended state;

FIG. 21 shows the actuator and displacement mechanism of FIG. 19 in apartially extended state;

FIG. 22 shows the actuator and displacement mechanism of FIG. 19 in itsfully extended state;

FIG. 23 is an exploded perspective view of a modification for a pumpapplication of the actuator and displacement mechanism illustrated inFIG. 19;

FIG. 24 is an exploded view of a pump employing the assembly of FIGS. 14to 18;

FIG. 25 shows the assembled pump of FIG. 24 on its inlet stroke; and

FIG. 26 shows the assembled pump of FIG. 24 on its outlet stroke.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1 to 14, the principles of operation of one form ofdisplacement mechanism according to the invention will now be described.This mechanism involves a stacked assembly 100 of discoidal elements,also referred to as discs, of which three, numbered 101, 102 and 103,are shown in FIG. 1. The discs are not planar but are relieved toprovide four planar ramp surfaces in four sectors on opposite sides,surrounding a central bore 104, those on one side being offset by 45°from those on the other side. Four of the ramped surfaces 105-108 areseen in the end elevation of the mechanism from the left hand side,looking at disc 101, as shown in FIG. 2. The visible edges of the rampedsurfaces on the assembly 100 are drawn as continuous lines in FIGS. 1 to4 whereas the invisible edges are dashed. The edges of the discs nearestthe viewer are hatched for illustrative purposes only. Each rampedsurface terminates in a steep return step, such as steps 109-112 in thecase of the outer face of disc 101.

In FIG. 1, the assembly 100 is in its unextended state and the rampedsurfaces of the three discs are snugly interengaged in complementaryfashion to take up the minimum space. If discs 101 and 103 are rotatedin the direction of the arrow shown in FIG. 2, while the intermediatedisc 102 is restrained against rotation, the camming action of theopposed ramped surfaces forces the discs to separate, as shown in FIG.3. The maximum displacement is achieved, as shown in FIG. 4, after arotation of 45°, when the stepped return surfaces at the end of theopposed ramp surfaces coincide. The maximum displacement is equal todouble the height of the ramps multiplied by the number of disc-to-discinterfaces and the rotation needed to achieve it depends on the numberof sectors per disc. So in this example, four sectors require a rotationof 45° to achieve maximum displacement.

How this principle is applied to a practical mechanism is illustrated inFIGS. 5 to 9. In FIGS. 1 to 4, no distinction was made between thediscs, except for the implied restraint against rotation of disc 102. Ina practical application, it is necessary to design driver and drivendiscs differently. In fact, in the assembly 120 of FIG. 5, there areseveral types of each disc. These consist of an input driver disc 121,identical intermediate driven discs 122 alternating with identicaldriver discs 123 and 124, and terminating in an output driven disc 125.The driver discs 123 and 124 are structurally identical but discs 123are in a first orientation while discs 124 are oriented at 90° to discs123. All the discs are stacked together in engagement with each other ona common axis.

Torque to rotate the input driver disc 121 is provided by way of anintegral threaded shaft 126 by means not shown in this drawing, such asa motor or a manual crank. In order for the mechanism to extend, thedrive torque must be transmitted from input driver disc 121 to all ofthe driver discs 123 and 124. Also the driven discs 122 must berestrained against rotation. This restraint is achieved by means of fourprojecting lugs 127 on each driven disc which can locate in an externalspline or similar channels, not shown in this drawing.

The communication of the drive torque cannot be by fixed linkage becausethe separation between the driver discs increases as the assemblyextends and they move outwardly along the axis. Communication of thetorque from driver disc to driver disc is thus effected by a system ofprojecting ribs 128, 129 which consist of internal raised portions,formed within keyhole bores 131 and 132 within central bosses 133 of thedriver discs, and external prong-like portions. The external prongs passthrough bores 130 in the driven discs and engage in the keyhole bores131, 132 of adjacent driver discs. The prongs 128 and bores 132 ondriver discs 123 are identical to the prongs 129 and keyhole bores 131on driver discs 124, the only difference being their relativeorientation of 90° to each other in the assembly stack.

Each projecting rib extends over 90° of arc so that its extending prongportions actually key into the spaces between the ribs in the centralbore of the next driver disc. Thus the driver discs 123, 124 are keyedfor rotation together and with the input driver disc 121 by means of theengagement of the pronged extensions of ribs 128, 129 with the internalportions of the ribs 128, 129 within keyhole bores 131, 132 of the nextdriver disc. At the same time, this arrangement of prongs and keyholesallows them to slide relative to each other in the axial direction,thereby enabling the assembly to extend.

FIGS. 7, 8 and 9 show the assembly 120 in its unextended, partiallyextended and fully extended states, respectively, the fully extendedstate again being achieved after a rotation of 90°.

Another form of displacement mechanism according to the further aspectof the invention is illustrated in principle in FIGS. 10 to 13 and apractical implementation is shown in FIGS. 14 to 18. FIGS. 10, 12 and 13show a stacked assembly 150 of three discoidal elements 151, 152 and153. For clarity, the outer edges of the discs are shown cross hatchedin FIGS. 10, 12 and 13. The operation of the mechanism is very similarto that of the mechanism of FIGS. 1 to 4, the principal difference lyingin the relief of the faces of the discoidal elements.

By way of example, one of the elements 152 is shown in perspective inFIG. 11 in the initial orientation that it has in FIG. 10. It can beseen, by noting the intersection of the disc at various points withthree dashed reference circles, that instead of a ramped surface, thedisc has smooth out-of-plane undulations, surrounding a central bore,154. Looking at the visible face of disc 152 from the right in FIG. 11,these undulations consist of three ridges, 155,156 and 157, interspersedwith three valleys, 158, 159 and 160. On the reverse face, the ridgesbecome valleys and vice versa. It should be noted that, although, inFIG. 11, the discs do not appear circular but waisted, this is an effectof the undulations on the perspective view and is caused by the factthat the ridges, such as 155, are raised with respect to theneighbouring valleys, 158 and 160. The vertical projection of a disconto a plane is actually still a circle.

FIG. 10 shows the assembly in its unextended state with the discs151-153 in a relative rotational orientation which takes up the minimumspace. In this orientation, the discs interfit snugly with theirundulating surfaces in full contact so that the ridges and valleys ofeach disc surface nestle in the valleys and ridges respectively of anadjacent disc surface. In this example, it is assumed that all the discsor at least discs 151 and 153 can move axially. It is further assumedthat disc 152 can be rotated while discs 151 and 153 are restrainedagainst rotation.

The effect of rotation of disc 152, as shown in FIG. 12 is to drive thediscs 151 and 153 away from disc 152 by camming action, as the risingslopes of the opposed surfaces bear on each other. Note the new positionof ridge 155 of driver disc 152, corresponding to a rotation of 30°.Ultimately, after a total rotation of 60°, as shown in FIG. 15, theassembly is fully extended with the ridges of the undulating discsurfaces aligned.

A practical assembly 161, operating according to the principles of FIGS.10 to 13 is shown in exploded perspective view in FIG. 14 and a portionof the assembly is shown enlarged in FIG. 15. Similarly to the steppeddisc version, the undulating disc stack is made up of a unique inputdriver portion, connected directly to threaded input drive shaft 162.The input driver portion is relieved on its inner face similarly todriver discs, 164

The driver discs 164 are all identical but have successively differentorientations in the stack. Each drive has a central bore 165. Identicaldriven discs 166 are located between each pair of driver discs. Thestack terminates in a driven output disc 167, seen on the right in FIG.14. This has a relieved inner face but a blank outer face fortransmitting linear output force.

Drive is communicated from the input drive shaft 162, via its driverportion to the driver discs 164 which are able to separate axially, bymeans of a system of prongs and keyholes similar to that of FIGS. 5 and6. However, because of the lack of depth of those discs, it is necessaryto have 4 pairs of shorter prongs 169 instead of the two shown on thestepped type. These are shown schematically in dashed outline in FIGS.14 and 15. As can be seen from the orientation of the prongs in thedrawing, successive driver discs are rotated by 90° with respect to thenext driver disc. The prongs pass through central bores in the drivendiscs and key into correspondingly shaped recesses in the bores 165 ofother driver discs and of the input driver portion on shaft 162. Becausethe discs are so thin, the prongs actually pass through and key intomore than a single neighbouring driver disc in the stack.

The driven discs 166 are each restrained against rotation by a system offour lugs 168, located 90° apart on the circumference of the drivendiscs. These engage in splined external channels, not shown in thisdrawing. As the driver discs are rotated, the assembly expands owing tothe camming action between driver and driven discs.

The assembly 161 is shown in FIGS. 16, 17 and 18 in its unextended,partially extended and fully extended states, respectively. Incomparison with the stepped disc assembly 120 of FIGS. 7 to 9, theextension is the same for the same amount of relief but it will be notedthat the discs of assembly 161 can be packed much more closely in theirunextended state. Thus a much more compact actuator can be producedusing the undulating discs or else a much greater extension can be usedby packing more discs into the same initial length assembly. Theseillustrations show how fewer undulating discs achieve the same offset asthe stepped version and could possibly achieve an offset of 200% oftheir initial unextended length.

Turning now to FIGS. 19 to 22, the incorporation of the assemblies 120or 161 into a complete rotary to linear displacement mechanism in anactuator will be described. FIG. 19 is an exploded view of the actuator,which has a common structure capable of accommodating either the steppeddisc assembly 120 or the undulating disc assembly 161, both of which areshown in their unextended state. In fully assembled form in FIGS. 20 to22, only the stepped disc version is shown but it will be understoodthat it is interchangeable with the undulating disc version. However,the following description will refer only to the stepped disc version,for brevity.

An annular base plate 170 supports the moveable portions of the actuatorby means of two bearing races 171 and 172 in which the drive shaft 126is mounted for rotation. A drive mechanism 173 comprises a hub 174,threaded onto shaft 126 which hub is itself rotated by a crank 175. Thedrive mechanism 173 could equally well be an electric motor such as astepper motor or servo motor.

The assembly 120 is housed in a cylindrical piston-like cover 180 whichis of the same length as the unextended assembly 120. At its open end,the cover 180 terminates in a flange 181, provided with four slots 182.These slots locate slideably on the exterior of four guide legs 183,secured to the base plate 170 at one end and bolted to a collar 184 atthe other end to form a cage for the piston cover 180. The cover 180 isfree to move axially and to protrude through the collar 184 when drivenby the expanded disc assembly. The other end of the piston cover isbolted to an end plate 185, for delivering the output of the actuator.To restore the actuator to its original state, that is, with theassembly contracted, a return spring 190 is located between the pistoncover flange 181 and the collar 184 to provide a resilient bias againstexpansion. The return spring is a compression spring and bears on theflange 181 and the collar 184.

In order to prevent the driven discs of the disc assembly from rotating,the lugs 127 of the driven discs in assembly 120 locate in narrowchannels 191 running along the piston cover 180 in an axial direction.Since, however, in its expanded state the disc assembly is much longerthan the cover 180, the guide legs 183 are also provided with furtherinternal grooves 192, aligned with grooves 191 on the piston cover.These grooves 191 and 192 ensure that the lugs 127 of driven discs 122are always engaged to prevent rotation.

The operation of the actuator can be better understood by looking atFIGS. 20 to 22. In FIG. 20, the actuator is in its unextended state.Operation of the crank 175 in the direction of the arrow rotates the hub174 and drive shaft 126 to cause expansion of the disc assembly 120.This forces the piston cover 180 outwardly against the action of thereturn spring 190, guided by guide legs 183, as shown in FIG. 21. InFIG. 22, the piston 180 is fully extended.

If the described actuator is to be used in applications requiring asingle stroke, such as precision positioning or dispensing of a measuredvolume of fluid, then it is desirable to limit the travel to prevent thediscs overshooting their maximum displacement.

The displacement of the actuator of FIGS. 19 to 22 is limited by theaction of the cover flange 181 fully compressing the spring 190 againstthe collar 184 as shown in FIG. 22. This stops rotation of the steppeddiscs beyond their maximum displacement, which would result in an abruptreturn as return steps 106 of adjacent discs slipped over each other. Ifthe piston cover were slightly longer, it would be possible to drive themechanism with a continuously rotating input and produce a reciprocatingmotion. Clearly, this would be smoother if the undulating disc assemblywere used, as this has equally smooth stroke and return slopes but thereturn stroke is faster with the stepped version.

In comparing the two types of disc, the major advantage of theundulating version is that it is particularly compact when unextendedand therefore is more suitable for applications having a limited space.

A variant of the assembly of FIGS. 19 to 22 which is more suitable for apump application is shown in FIG. 23. This is largely identical to FIG.19, identical parts being identically numbered, but includes a largerbase plate 197, a secondary piston cover 193, in place of end cap 185and an outer casing 194 in which sits an O-ring 195. The secondarypiston cover 193 slides up past the O-ring, secured in the recess at theend of the outer casing 194 and is restrained from over extension by aflange 196 at the foot of the outer piston cover. The secondary pistoncover is thus able to pump fluid without leakage from the cylinderformed by the outer casing.

FIG. 24 illustrates the application of the undulating disc assembly asdescribed in FIGS. 14 to 18 to a pump. The assembly 161 is mounted in apump barrel 200 and driven against a compression spring 201 at the endof the barrel having an outlet valve 202. The inside of the barrel issplined or grooved to constrain the driven discs of the assembly tolinear movement only, by engagement of lugs 168 with the grooves.

The assembly is driven, in a similar manner to FIG. 19 by means of acrank handle 203 and hub 204. The hub 204 and the input drive shaft 162are mounted in bearings 205 located in a threaded end cap 206 at theopposite end of the barrel to the outlet valve. Because the barrel islong enough to permit the discs of assembly 161 to rotate continuously,the assembly expands and contracts in reciprocating fashion to producethe pumping action. A disc 207 acts as a one way seal to permit air orother pumped fluid to be drawn into the outlet end of the pump barrel.

Although the stepped disc mechanism could also be used, the undulatingversion offers a smoother reciprocating action, albeit with a slowerreturn action.

FIG. 25 shows the assembled pump at one extreme of its inlet stroke,with the assembly 161 fully contracted. FIG. 26 shows the assembled pumpat the extreme of its outlet stroke, with the assembly 161 fullyextended.

1. A rotational to linear displacement conversion mechanism comprising:an assembly including a plurality of driver discoidal elements (101,123,164) and a plurality of driven discoidal elements (102, 122,166) mountedalternately on a common central axis to form an interleaved stack, eachdiscoidal element having a ramped surface (105-108, 155-157), the rampedsurfaces of adjacent elements being complementarily shaped and opposedso that, when in contact and completely interengaged, the discoidalelements form a stack of minimum length; coupling means (128, 129, 131,132) for coupling the driver discoidal elements for rotation togetherabout the axis by an externally applied force while permitting them totranslate along the axis; a driven element mounting means wherein saiddriven elements are mounted in such a way as to permit translation ofthe driven elements along the common axis while preventing rotation ofthe driven elements about the common axis, whereby a rotationaldisplacement of the driver elements by the externally applied forcecauses the elements to separate by camming action of the interengagedramp surfaces so as to produce an extension of the stack correspondingto the cumulative separations of the driver and driven elements; andresilient bias means (190) for restoring the assembly to its minimumlength in the absence of the externally applied force.
 2. The mechanismas claimed in claim 1 in which at least the driven discoidal elementsintermediate the ends of the stack are provided with axially alignedbores (130), each driver element having a projection, extending axiallyfrom one face, which extends through the bore of a driven disc adjacentto the projection and locates in a recess in a recess in a proximatedriver element in keyed, slideable engagement therewith so thatrotational drive force can be transmitted between driver elements whileallowing relative sliding motion in an axial direction.
 3. The mechanismas claimed in claim 2, in which each said driver element recess is partof a bore (131, 132) through the driver element and said projection(128, 129) is part of at least one rib formed on an inner surface of thebore of said projection's corresponding driver element, wherein the atleast one rib projects outwardly from said at least one rib's driverelement and engages at least one complementarily oriented rib portion inthe bore of the proximate driver element to provide said keyed slideableengagement.
 4. The mechanism as claimed in claim 3, in which the bore(131,132) in each intermediate driver element is provided with twodiametrically opposite ribs each extending over a 90 degree arc of thebore, said ribs being keyed into engagement with a similar pair of ribsin the proximate driver element oriented at 90 degrees to the firstmentioned pair of ribs.
 5. The mechanism as claimed in claim 1, in whichthe driven elements each have a plurality of peripheral lugs (127; 168),the driven element mounting means including a grooved guide means(180,191, 183, 192) surrounding the stack in which the lugs locate toprevent rotation.
 6. The mechanism as claimed in claim 1, in which aninitial driver element (121) is located at one end of the stack and hasan out portion, adapted to be coupled to an external drive, and an innerramped surface and in which a terminal driven element (125) is locatedat the opposite end of the stack and has an outer surface, adapted todeliver a translational load force, and an inner ramped surface,intermediate driver and driven elements having ramped surfaces on bothsides.
 7. The mechanism as claimed in claim 6, in which the drivenelements each have a plurality of peripheral lugs (127; 168), the drivenelement mounting means including a grooved guide means (180,191, 183,192) surrounding the stack in which the lugs locate to prevent rotationand which includes a housing assembly for the stack, said housingassembly forming said driven element mounting means and comprising acylindrical cover (180) for the stack to one end of which the terminaldriven element is fixed, the other end of the cover terminating in aslotted flange (181), the housing assembly further comprising a fixedcage structure surrounding the cylindrical cover and being formed with aplurality of guide legs (183) extending in the axial direction andpassing through the slots (182) in the flange of the cylindrical coverto constrain the cylindrical cover to linear movement, the cylindricalcover being provided with external grooves (191) and the guide legsbeing provided with internal grooves (192), in both of which saidperipheral lugs locate in operation to restrain the driven discs againstrotation while permitting translation.
 8. The mechanism as claimed inclaim 7 in which the resilient bias means is a coil spring (190) trappedbetween the flange (181) of the cylindrical cover and an end (184) ofthe cage.
 9. The mechanism as claimed in claim 6, in which said initialdriver element (121) is fixedly mounted on an outwardly extending axialshaft (126), threaded externally for coupling to the external drive. 10.The mechanism as claimed in claim 1, in which the driver and drivenelements each have a plurality of ramps (105-108) per ramped surface,distributed circumferentially at evenly spaced positions.
 11. Themechanism as claimed in claim 10, in which the camming ramp surfaces(105-108) are planar rising at a relatively shallow angle to the planeof the discoidal elements and alternating with relatively steep returnsurfaces (109-111).
 12. The mechanism as claimed in claim 10, in whichboth rising and falling surfaces of the ramps are at a same angle. 13.The mechanism as claimed in claim 10, in which the ramped surfaces aresmoothly undulating in form without discontinuity at peaks (155-157).14. The mechanism as claimed claim 1, including a stop for preventingrotation of the driver elements beyond an arc defined by the rampsurfaces.
 15. An actuator comprising: a rotational to lineardisplacement conversion mechanism comprising: an assembly including aplurality of driver discoidal elements (101,123, 164) and a plurality ofdriven discoidal elements (102, 122,166) mounted alternately on a commoncentral axis to form an interleaved stack, each discoidal element havinga ramped surface (105-108, 155-157), the ramped surfaces of adjacentelements being complementarily shaped and opposed so that, when incontact and completely interengaged, the discoidal elements form a stackof minimum length; coupling means (128, 129, 131, 132) for coupling thedriver discoidal elements for rotation together about the axis by anexternally applied force while permitting them to translate along theaxis; a driven element mounting means wherein said driven elements aremounted in such a way as to permit translation of the driven elementsalong the common axis while preventing rotation of the driven elementsabout the common axis, whereby a rotational displacement of the driverelements by the externally applied force causes the elements to separateby camming action of the interengaged ramp surfaces so as to produce anextension of the stack corresponding to the cumulative separations ofthe driver and driven elements; and resilient bias means (190) forrestoring the assembly to its minimum length in the absence of theexternally applied force; and a drive mechanism (173) for rotatablydriving the driver elements.
 16. The actuator as claimed in claim 15 inwhich the drive mechanism is a motor.
 17. The actuator as claimed inclaim 15 in which the drive mechanism is a manually operated crank(175).
 18. The actuator as claimed in claim 15 in which the displacementconversion mechanism is adapted for continuous rotation of the driverelement.
 19. A pump including: a rotational to linear displacementconversion mechanism comprising: an assembly including a plurality ofdriver discoidal elements (101,123, 164) and a plurality of drivendiscoidal elements (102, 122,166) mounted alternately on a commoncentral axis to form an interleaved stack, each discoidal element havinga ramped surface (105-108, 155-157), the ramped surfaces of adjacentelements being complementarily shaped and opposed so that, when incontact and completely interengaged, the discoidal elements form a stackof minimum length; coupling means (128, 129, 131, 132) for coupling thedriver discoidal elements for rotation together about the axis by anexternally applied force while permitting them to translate along theaxis; a driven element mounting means wherein said driven elements aremounted in such a way as to permit translation of the driven elementsalong the common axis while preventing rotation of the driven elementsabout the common axis, whereby a rotational displacement of the driverelements by the externally applied force causes the elements to separateby camming action of the interengaged ramp surfaces so as to produce anextension of the stack corresponding to the cumulative separations ofthe driver and driven elements; and resilient bias means (190) forrestoring the assembly to its minimum length in the absence of theexternally applied force; a rotatable crank (203) for rotatably drivingthe driver elements; and a sealed casing (200) for enclosing themechanism and forming a pump chamber containing a one way inlet means(207) for permitting fluid to be drawn into the pump chamber as themechanism contracts and an outlet (202) for enabling fluid to beexpelled from the pump chamber as the mechanism extends one way, as thecrank is rotated continuously.
 20. A pump as claimed in claim 19 inwhich the driver and driven elements each have a plurality of ramps(105-108) per ramped surface, distributed circumferentially at evenlyspaced positions.